The 35-year-old Argentina star had been strongly linked with a move to Saudi Arabian club Al Hilal, to join a league which already features Cristiano Ronaldo and now Karim Benzema.
However, with Messi's two-year contract at Paris St Germain coming to an end this month, the World Cup winner confirmed in a joint interview with Spanish publications Mundo Deportivo and Sport he was set to head to the United States.
"I made the decision that I am going to go to Miami. I still don't have it 100 per cent closed or something is missing, but well, we decided to continue the path there," he told Spanish sport news website Mundo Deportivo in an interview on Wednesday.
The Florida side recently sacked Beckham's former Manchester United and England team-mate Phil Neville as coach and are currently bottom of the 15-team Eastern Conference, but remain a fiercely ambitious club.
Messi had also been linked with a move back to his former club Barcelona, but confirmed when that was not set to go though, he opted for a fresh challenge overseas.
"If the Barcelona thing didn't work out, I wanted to leave Europe, get out of the spotlight and think more in my family," the Argentina playmaker said.
"I really wanted, I was very excited to be able to return (to Barcelona), but after having experienced what I experienced and the exit I had, I did not want to be in the same situation again - waiting to see what was going to happen.
"I heard that they had to sell players or lower players' salaries and the truth is that I didn't want to go through that, nor take charge of obtaining something that had to do with all that."
The PA news agency understands the Florida side pulled out all the stops to sign Messi, tying up with various MLS partners like Apple.
with PA
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